Although the Hubble Space Telescope has an aperture one-half that of the 5-m Hale Reflector and only one-quarter that of the 10-m Keck reflector this extraordinary orbiting telescope has returned images with resolutions notably superior to those presently attainable from significantly larger terrestrial telescopes. This performance affords a glimpse at the science potential for orbiting telescopes with significantly greater apertures than even the 10-m Keck when free of the deleterious effects of atmospheric absorption and turbulence. However in terms of practical launch requirements and orbital erection necessities, as well as economic realities, the the conventional telescope optical arrangement based on the monolithic circular aperture, an arrangement astronomers have relied on since Galileo and Newton, is not particularly suitable for telescopes significantly larger than the Keck. Even for terrestrial telescopes viability may not extend much beyond the Keck aperture, if in fact a 10-m monolithic aperture can be fabricated. Nevertheless the need for significantly greater resolving power than presently available is imperative. For example, consider the diffraction-limited resolution of a simulated 15th magnitude quasar possible with 2.5-m and 8-m apertures, as shown in FIG. 1. These resolutions are compared to that possible with a 25-m aperture, an aperture extent simply outside the realm of present technology in regard to the circular aperture.
Beyond 10 meters rigid monolithic mirrors become essentially impractical, if not from practical fabrication, transport and erection considerations, then from the significant gravitational gradient distortions and temperature variations across the 5-meter extent of the mirror would ruinously distort the intrinsic figure without corrective measures, whether the instrument were terrestrial or orbital.
In a significant departure from the monolithic aperture the Keck 10-m reflector relies on an array of hexagonal mirrors. The Hubble 2.4-m and Hale 5-m reflectors are shown for comparison with the Keck in FIG. 2 so as to appreciate the enormity of any projected 25-m instrument. Because final figuring of the Keck is accomplished by distortion of the individually figured mirrors using servomechanical actuators, temperature and gravitational distortions can be compensated for. However the focusing and aligning algorithms for compensation are highly complex. Moreover the mirrors must be first mechanically figured, a considerable difficulty because of their non-symmetric configuration and non-circular perimeter. Nevertheless future reliance ostensibly is to be placed on the sectored-mirror circular aperture for very large terrestrial and orbital telescopes.
Accordingly the Keck configuration would be scaled up from a 10-meter to a 25-meter aperture. Two basic sectored mirror arrangements that represent extremities in the Keck configuration can be considered. A 36-sector design would be essentially the existing Keck reflector simply scaled up to 25 meters with each sector mechanically figured and servomechanically distorted for final figuring. However the mirrors would be particularly difficult to figure for a 25-m aperture with each sector significantly larger than the Hubble's mirror.
Alternatively, an equivalent aperture could be attained using 176 Keck-size sectors, each mechanically figured and individually distorted. However, because of practical mechanical and electronic limitations on the compensating system required for just 36 Keck-size mirrors, the full imaging potential of the Keck is rarely achieved in practice, and would be essentially unachievable for 176 sectors. Hence, for all practical purposes apertures significantly larger than the Keck 10-m are beyond present technology. In light of these difficulty alternative arrangements are being considered, all involving partially-filled apertures, still based on the circular aperture however.